Refine checkstyle

[koppor]

Since my last checkstyle PR #6283 new versions of checkstyle have been released. I went through the changelog (https://checkstyle.sourceforge.io/releasenotes.html) and added new interesting rules. Here the result.

• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.